Conventional longwave infrared (“LWIR”) lenses are expensive to manufacture and often incorporate either moving parts or nested barrels for athermalization. Athermalization is a larger concern for infrared (“IR”) imaging systems than for systems that image visible radiation, due to the relative increased sensitivity to temperature of common LWIR materials in comparison to common visible spectrum materials. FIG. 1 illustrates one prior art manufacturing process 100 for producing a camera assembly. In process 100, aspheric lens elements are diamond turned individually in step 102, and a housing is fabricated in step 104. The lenses are placed into a lens mount with multiple component athermalization mechanisms in step 106. The lens mount is assembled into a camera in step 108. The camera is actively aligned and focused using feedback from acquired images, and then potted into place, in step 110. The camera is calibrated and tested in step 112.
FIG. 2 illustrates, in an exploded view, one prior art lens stack 200 that may be produced by steps 102 through 108 of process 100 of FIG. 1. Lenses 202 and 206 are diamond turned in step 102 of process 100. Housing 216 is fabricated in step 104. In step 106, lenses 202 and 206 mount with a spacer 204 therebetween and with a first barrel 210 and a second barrel 212 that has a high coefficient of thermal expansion. These components then mount with a threaded barrel 214 that has a low coefficient of thermal expansion. In step 108, threaded barrel 214 screws into housing 216. Lens stack 200 may integrate with a sensor 218 to form a camera.
As shown in FIG. 2, lens stack 200 includes a significant number of associated mounting hardware elements and interfaces between these elements and lenses. Those skilled in the art will appreciate that these elements generate tolerance stack-up issues (e.g., related to lens alignment, element thickness variations, tilt of the elements and mounting materials) that can make the performance of lens stack 200 nonideal. Therefore, lens stack 200 is usually actively aligned and focused in situ with a camera body in step 110 to mitigate the effects of these issues, but the active alignment process adds further cost to process 100.
Another prior art method of making cameras includes molding lenses onto one or both sides of a transparent glass substrate. Multiple wafers of lenses are then stacked on top of each other with spacer wafers between the lens wafers to achieve a required spacing between the lenses. Good imaging performance of the final lens assemblies requires precise positioning of all of the wafers in six degrees of freedom with respect to each other: typical Cartesian x and y coordinates for centering the lens elements; z spacing between the lens elements, and rotations known in the art as tip, tilt and theta. The required alignments are generally performed utilizing a mask aligner adapted from semiconductor processing equipment. Such equipment may be costly and time consuming to operate, and presents special challenges for assembly of IR optics. IR imaging instrumentation would generally be a nonstandard and costly addition to a mask aligner, and the longer wavelengths of IR as opposed to visible light may make alignment thereby less precise.